Further examples of potential biases

The following list of other potential sources of bias in a clinical study may aid detection of further problems.


Inappropriate influence of funders (or, more generally, of people with a vested interest in the results) is often regarded as an important risk of bias. For example, in one empirical study, more than half of the protocols for industry-initiated trials stated that the sponsor either owns the data or needs to approve the manuscript, or both; none of these constraints were stated in any of the trial publications (Gøtzsche 2006). It is important that information about vested interests is collected and presented when relevant. However, review authors should provide this information in the ‘Characteristics of included studies’ table (see Section 11.2.2). The ‘Risk of bias’ table should be used to assess specific aspects of methodology that might be been influenced by vested interests and which may lead directly to a risk of bias. Note that some decisions that may be influenced by those with a vested interest, such as choice of a particularly low dose of a comparator drug, should be addressed as a source of heterogeneity rather than through the ‘Risk of bias’ tool, since they do not impact directly on the internal validity of the findings.